A PRELIMINARY ASSESSMENT OF SUBAQUEOUS TAILINGS DISPOSAL IN ANDERSON LAKE, MANITOBA MEND Project 2.11.1a-c March 1990 A Preliminary Assessment of Subaqueous Tailings Disposa1 in Anderson Lake, 6hanitoba Prepared for and Funded by: British Columbia Ministry of Energy, Mines and Petroleum Resources CANMET Environment Canada Hudson Bay Mining and Smelting CO., Ltd. Prepared Sy: March 1990 Rescan Environmental Services Ltd. Vancouver, Canada EXECUTIVE SUMMARY A preliminary field assessment of the effects of subaqueous disposa1 of tailings in Anderson Lake, Manitoba was conducted as part of the Mine Environment Neutral Drainage (MEND) program. The lake has received about 7.5 million tonnes of tailings since 1979 from the Snow Lake mil1 operated by Hudson Bay Mining & Smelting CO., Ltd. The mil1 processes copper-lead-zinc ore from four nearby underground mines. Tailings are discharged into the lake through a floating pipe, which is regularly moved to prevent tailings buildup. The lake is a small, Precambrian Shield waterbody, with high biological productivity (meso to eutrophic conditions). Historically, it has been considered a dead lake due to the limited fisheries resources. The lake is essentially land-locked with minimal inflows and outflows. A dam with a control discharge structure has been installed at the outlet, and has increased lake area. However, due to tailings deposition, the overall lake volume has decreased. The lake is shallow (mean depth = 2.1 m) with a maximum depth of 6.7 m. No thermocline was observed in the water column, but dissolved oxygen concentrations were markedly decreasedwithin 0.5 m of the bottom. The lake is characterized by higher conductivity, dissolved solids, sulphate concentrations and increased hardness since tailings discharges have begun. The pH has generally decreased. Dissolved metal concentrations and turbidity have increased in the area near the discharge. Inflows from the Anderson mine area are characterized by low pH, high conductivity and dissolved solids, low alkalinity, high sulphate and reactive silicate concentrations, higher chloride concentrations and high dissolved metal concentrations. Metal concentrations in the lake sediments have been greatly increased. The sediment stations near the outfall show metal levels comparable to fresh tailings samples. Stations where tailings have been deposited in the past are developing an organic layer that is biologically active. Detailed petrographic, X-ray diffraction @RD), X-ray fluorescence (XRF), particle size and leaching analyseswere performed to characterize the sediment and tailings. -i- EXECUTIVE SUMMARY Sediments were characterized into two major groups, those predominantly composed of tailings materials (the tailings, and Stations 1,2, and 4) and those primarily composed of organic material (Stations 3,5 and 7). XRD work indicated higher abundance of quartz, feldspar, pyrite, micas and calcite in the tailings dominated samples. The actual tailings were composed of 55 % pyrite, 4% pyrrhotite, 2% sphalerite and minor portions of hornblende, biotite, chlorite, carbonate and other silicates. Minor to trace amounts of chalcopyrite, arsenopyrite and galena were found. The amount of tailings estimated in the organic samples varied with the method used. The organic samples contained framboidal pyrite and fine specks of angular pyrite, while tailings dominated samples have similar compositions and grain size to the tailings and were virtually unmodified. Minera1 compositions assumed from XRF analyses indicated the presence of various concentrations of quartz, plagioclase feldspar, micas, chlorite, and pyrite. The highest percentages of pyrite were found in the tailings and tailings-dominated samples. Sequential extractions of a tailings-dominated sample (Station 2) indicated generally low metals availability, with most of the metals associated with the oxidizable and residual phases. These observations are in agreement with the high sulphide content of the sample. The results suggest that the metals are generally non-labile and that significant metal release would require strongly oxidizing and acidic conditions to exist. Present conditions in Andersen Lake are not conducive to such release; hence, the potential for metal transfer from sediments to the aqueous environment, in levels that are toxic, is considered to be minimal. With exception, however, was the behaviour of zinc. Due to the high total Zn concentrations, low releases of Zn occurring in the other phases indicated a potential for detectable release under natural conditions. Cadmium, copper, lead and nickel were also released In low quantities in phases other than the oxidizable and residual phases, Acid-base accounting of the tailings sample indicated a considerable potential for acid generation. The biota of the lake were examined in considerable detail. Densities of benthic invertebrates were low but comparable to previous studies both in number and species composition. Phytoplankton densities and community structure characterize the lake as mesotrophic. Examination of past phytoplankton communities indicates that more eutrophic conditions existed prior to tailings deposition. Zooplankton densities were highly variable, but were generally lower near the tailings discharge. Zooplankton diversity is comparable to that observed in other Manitoba lakes. Aquatic vegetation of the lake was mapped and samples collected for metal analyses. Metal levels in Typha were generally highest in a tailings seepage area, particularly for arsenic, cadmium, lead and zinc. Outside of the seepage area, concentrations of metals were low except for nickel. Fish sampling confirmed that the fishery resources was limited, as only brook sticklebacks (Culaea incomtam) were caught. Tissue- concentrations in the stickleback suggest bioaccumulation of copper, lead and zinc. Metal levels in the fish were also significantly higher in populations from the tailings deposition area. - III - SOMMAIRE Dans le cadre du Programme de neutralisation des eaux de drainage dans l’environnement minier (NEDEM), les répercussions de l’élimination subaquatique de résidus miniers dans le lac Anderson au Manitoba ont fait l’objet d’une évaluation sur le terrain. Depuis 1979, environ 7.5 million de tonnes de résidus miniers ont été déversés dans ce lac par l’usine Snow Lake exploitée par la Compagnie minière et métallurgique de la Baie d’Hudson Ltée. Cette usine sert au traitement du minerai de cuivre/plomb/zinc tiré de sept mines souterraines du voisinage. Les résidus sont rejetés dans le lac au moyen d’un tuyau flottant, qui est déplacé régulièrement de façon à empêcher leur accumulation. Ce lac, qui est une petite pièce d’eau du Bouclier précambrien, possède une activité biologique élevée (les conditions y sont mésotrophes ou eutrophes). Il y a longtemps qu’il est considéré comme mort vu le peu de poissons qu’on y rencontre. Le renouvellement de ses eaux est nul car il est entouré de terres et les débits d’entrée et de sortie sont minimaux. A la sortie du lac, on a construit un barrage permettant d’en régler le débit, ce qui a augmenté sa superficie. Toutefois, la sédimentation des résidus miniers a entraîné une diminution de son volume global. Ce lac est peu profond : en moyenne, 2.1 m et au maximum, 6.7 m. Aucune thermocline n’a été observée dans la colonne d’eau, mais on a noté que la concentration d’oxygène dissous augmentait d’une façon marquée à 0,5 m du fond. Depuis qu’on a commencé d’y déverser des résidus miniers, ce lac se distingue des autres par une conductivité, des concentrations de matières solides dissoutes et de sulfates, et une dureté accrues; son pH a, en général, baissé; les concentrations de métal dissous et la turbidité ont augmenté près de la sortie. Les eaux provenant de la zone de la mine Anderson sont caractérisées par un pH bas, une conductivité élevée, une faible alcalinité et de fortes concentrations de matières solides dissoutes, de silicates réactifs, de sulfates et de métaux dissous, et des concentrations de chlorures supérieures. Les concentrations de métaux dans les sédiments lacustres ont beaucoup augmenté. Au poste d’échantillonnage situé près de la décharge, les teneurs en métaux des sédiments sont comparables à celles des échantillons de résidus miniers frais. Aux postes où les dépôts miniers datent de longtemps, une couche organique biologiquement active est en train de se former. Des études pétrographiques .et des analyses détaillées ont été effectuées pour caractériser les sédiments et les résidus miniers, tant par diffraction que par fluorescence des rayons X (DRX et FRX) et aussibien d’après la taille des particules que les constituants des lixiviats. Les sédiments ont été répartis en deux groupes principaux : ceux constitués surtout de matières appartenant aux résidus (les résidus eux-mêmes et les échantillons prélevés aux postes 1, 2 et 4) et ceux composés principalement de matières organiques (postes 3, 5 et 7). Des études par DRX ont montré des teneurs plus élevées en quartz, en feldspath, en pyrite, en micas et en calcite dans les échantillons constitués surtout de résidus. Les résidus eux-mêmes se composaient de pyrite (55 %), de pyrrhotite (4 %), de sphalérite (2 %) et, en faibles proportions, de homblende, de biotite, de chlorite, de carbonates et d’autres types de silicates. On a aussi trouvé de faibles quantités jusqu’à des traces de chalcopyrite, d’arsénopyrite et de galène. L’évaluation des quantités de résidus présentes dans les échantillons organiques varient selon